Crystal meth is turning brown trout into drug addicts, a study has found. 

The drug, also known as methamphetamine, is increasingly polluting waterways and researchers are investigating the impact it has on marine life.

But while recent research has found prescription drugs are increasing in concentration in rivers and streams, so too are illicit substances. 

Researchers from the Czech Republic took 120 juvenile brown trout that were bred in captivity and kept them in two different tanks, each containing 60 fish and 350 litres of water. 

One tank was then laced with methamphetamine to a concentration of one microgram per litre, a level often found in freshwater rivers in Europe and the UK. 

Some water systems in the world have been found to be poisoned with amphetamine levels 25 times this.

The fish in the experiment stayed in their tanks for eight weeks before they were transferred to a different, drug-free aquarium. 

Researchers then gave the fish the choice of staying in clean water, or returning to a drug-riddled tank. 

The theory, the researchers said, was that if the fish were suffering with symptoms of withdrawal they would opt to return to the methamphetamine water.

What they saw was a clear preference by the exposed fish for the contaminated waters as they suffered withdrawals during the first four days after moving to freshwater.

“Controls spent 41.5 per cent of all observations in the methamphetamine-dosed part of the arena,” wrote the study authors in their paper, published in the Journal of Experimental Biology.

A handful of crystal meth. Addicted fish were also found to be less active than trout that had never experienced the drug

Credit: Daniel Roland/AFP

The figure for the exposed brown trout however, was much higher at 50.5 per cent. 

Researchers also took samples from the brains of the fish to see how far the drug had permeated.

“Methamphetamine preference was positively correlated with levels of amphetamine residues in fish brains, suggesting that addiction is linked to the presence of this drug metabolite in nervous system tissue,” the researchers said. 

“Amphetamine was only identified in brain tissue of exposed trout.”

They added that every single one of the 60 fish in the meth-laden tank had signs of the drug in their brain after eight weeks. However, this dropped to one in eight after 10 days in clean water.

Addicted fish were also found to be less active than trout that had never experienced the drug.

How human rubbish tricks animals

Dr Pavel Horký, co-author of the study from the Czech University of Life Sciences Prague, also believes the drug addiction could drive fish to congregate near unhealthy water treatment discharges in search of a fix, as well as disturbing their natural tempo of life. 

“Such unnatural attraction to one area together with documented changes in behaviour could result in unexpected ecological consequences influencing whole ecosystems,” he suggested.

“The elicitation of drug addiction in wild fish could represent another example of unexpected evolutionary selection pressure for species living in urban environments along with ecological side effects of human societal problems within aquatic ecosystems.”

Drugs reach waterways through a variety of routes. Improper disposal, such as being washed down a sink or toilet, is one, while the drugs can also invade the habitat of fish via the urine of people taking them, which re-enters the water system despite being filtered.

A separate study recently found antidepressants are putting crayfish at increased risk of predators by making them more brazen. The crustaceans were found to spend more time searching for food – thereby putting themselves in danger – when water was polluted by Prozac.

“Crayfish exposed to the antidepressant came out into the open, emerging from their shelter more quickly than crayfish not exposed to the antidepressant," said Dr Lindsey Reisinger, a co-author of the study from the University of Florida.

“This change in behaviour could put them at greater risk of being eaten by a predator.”